Internal instability is a form of internal erosion in broadly graded
cohesionless soils in which fine particles can be eroded at lower
hydraulic gradients than predicted by classical theory for piping or
heave. A key mechanism enabling internal instability is the formation of
a stress-transmitting matrix dominated by the coarse particles, which
leaves the finer particles under lower effective stress. In this study,
discrete element modeling is used to analyze the fabric and effective
stress distribution within idealized gap-graded samples with varying
potential for internal stability. The reduction in stress within the
finer fraction of the materials is directly quantified from grain-scale
data. The particle-size distribution, percentage finer fraction, and
relative density are found to influence the stress distribution. In
particular, effective stress transfer within a critical finer fraction
between 24 and 35% is shown to be highly sensitive to relative density.